Best Practice in Best Pra ctice in teel constr uction - onstr industrial B uildings esidential Buildings 01 Introduction 02 Key Design Factors

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2 Best Practice in Steel Construction - Industrial Residential Buildings Contents The Steel Construction Institute (SCI) develops and promotes the effective use of steel in construction. It is an independent, membership based organisation. SCI s research and development activities cover multi-storey structures, industrial buildings, bridges, civil engineering and offshore engineering. Activities encompass design guidance on structural steel, light steel and stainless steels, dynamic performance, fire engineering, sustainable construction, architectural design, building physics (acoustic and thermal performance), value engineering, and information technology. This publication presents best practice for the design of steel construction technologies used in residential buildings, and is aimed at architects and other members of the design team in the early stages of planning a residential building project. It was prepared as one of a series of three under an RFCS dissemination project Euro-Build in Steel (Project n RFS2-CT ). The project s objective is to present design information on best practice in steel, and to take a forward look at the next generation of steel buildings. The other publications cover best design practice in commercial and residential buildings. The Euro-Build project partners are: ArcelorMittal Bouwen met Staal Centre Technique Industriel de la Construction Métallique (CTICM) Forschungsvereinigung Stahlanwendung (FOSTA) Labein Tecnalia SBI The Steel Construction Institute (SCI) Technische Universität Dortmund Although care has been taken to ensure, to the best of our knowledge, that all data and information contained herein are accurate to the extent that they relate to either matters of fact or accepted practice or matters of opinion at the time of publication, the partners in the Euro-Build project and the reviewers assume no responsibility for any errors in or misinterpretations of such data and/or information or any loss or damage arising from or related to their use. ISBN The Steel Construction Institute. This project was carried out with financial support from the European Commission s Research Fund for Coal and Steel. Front cover: Liljeholmstorget (Stockholm, Sweden). Photograph by JM AB. 01 Introduction 1 02 Key Design Factors 2 03 Floor Systems 04 Wall Systems Primary Steel Frames Modular Systems Façade & Roof Systems National Practice Case Studies 55

3 Introduction Introduction The design of housing and residential buildings is influenced by many factors, including new requirements for sustainability, and thermal and acoustic performance. The environmental need to conserve land use, whilst improving the social characteristics of the built environment, also have a direct effect on the choice of constructional system. The pressure for more efficient and sustainable construction processes to meet these challenges has led to a demand for higher degrees of prefabrication and improved quality in the performance of the chosen construction technology. Steel constructional technologies have achieved a high market share in other building sectors and the same technologies can be used in housing and residential buildings, where the main benefits include: speed of construction, higher levels of quality, reliability and longevity, and the ability to provide more adaptable use of space. This publication presents best practice for the range of steel technologies used in housing and in residential buildings of all types, including mixed use commercial and residential buildings. The steel technologies may be used separately or in combination to provide complete building systems. These hybrid forms of construction technologies lead to a wide variety of design solutions. The guidance covers structural and building physics aspects of these steel technologies. Differences in national practices are also identified and the constructional technologies are illustrated by a series of case examples of recent housing and residential projects in four countries. Figure storey terraced housing using light steel framing (Basingstoke, UK) HTA Architects

4 02 Best Practice in Steel Construction - Residential Buildings 02 Key Design Factors The design of housing and residential buildings is influenced by many factors. The following general guidance is presented to identify the key design factors and the benefits of steel construction in this sector. Housing & Residential Building Market New house building accounts for less than 1% of the total housing stock across Europe, but this sector of construction is the focus for improvements in performance and greater concern for sustainability in social, economic and environmental terms. Residential buildings are responsible for 27% of all CO 2 emissions across the EU and therefore are targeted for improvements in energy efficiency. The renovation of housing, including extending and adapting existing buildings, represents a significant additional market. There are important trends affecting the housing and residential building sector that are similar across Europe: Improved levels of thermal insulation and incorporation of renewable energy technologies to reduce primary energy use in this sector. Building to higher densities, particularly in urban locations, or on former industrial sites, in order to conserve land use. Building faster with less disruption and to higher quality, using prefabricated construction techniques. Reducing building costs and longterm operational costs. Increasing extent of single person and old person accommodation, reflecting changes in social patterns. Provision of buildings that are adaptable to a range of uses, and change of use in the longer term. Steel construction systems are well placed to respond to these trends, particularly when using prefabricated technologies and in the medium to highrise residential building sector, where speed of construction is more important. Increasingly, there is a trend towards mixed use buildings which may involve commercial, social and residential parts to create a live, work, play environment. Long-term flexibility in use and future adaptability is important in many building types. In housing, three storeys rather than two storeys are increasingly preferred in urban locations in order to minimise the building footprint and land use. An additional floor can be provided by efficient use of the roof space, which can be more readily achieved with open roof systems in steel. Kitchens and bathrooms can be produced as modular components in order to optimise the speed of construction and economy of scale in manufacture. Sustainability Environmental and sustainability issues dominate the design of new housing and residential buildings. Various national requirements exist for thermal performance and sustainability, which are embodied in national Regulations. These general sustainability issues may be characterised by specific requirements to: Reduce primary energy use and hence CO 2 emissions. Minimise materials use and waste, and maximise recycling of waste in construction. Housing and Residential Building Market Sustainability Speed of Construction Long-term Use Acoustic Insulation Fire Safety Thermal performance Loading

5 Key Design Factors 02 Figure 2.1 Apartment building in Helsinki showing use of integral balconies Kahri Architects Use water efficiently and make provision for recycling of grey water. Eliminate pollution and protect the local environment. Design of attractive public space and improved health and wellbeing in the building environment. Steel technologies score well in terms of these sustainability issues. For example, steel is 100% recyclable and the small amount of waste that is created in manufacture and construction is recycled. All steel construction systems can be re used or recycled at the end of their life. Prefabrication of steel components increases site productivity and speed of construction by up to 70%, and reduces the disruption to the locality during the construction process. Using steel construction, more adaptable space can be created, which leads to long life buildings that can serve a number of functions and future uses. Speed of Construction A characteristic of all steel technologies is their speed of construction on site and improved productivity through efficient construction using prefabricated systems. Studies have shown that 2 dimensional or panelised systems are 30 to 40% faster to build than masonry construction, for example, and that fully modular systems are 60 to 70% faster than these more traditional methods. The financial benefits of speed of construction are: Reduced site facilities and management costs. Early return on the client s investment. Reduced interest costs during the construction period. These benefits lead to reduced cash flow and higher return on capital. Speed of construction is particularly important for larger residential buildings and for buildings such as student residences, which must be completed to meet the academic year. Long-term Use The design life of housing and residential buildings is typically 60 years in terms of the primary structure and building envelope. However, buildings must be flexible in use and adaptable to future demands, which steel technologies can achieve through use of re-locatable partitions, longer span floors and use of open roof systems. Galvanised steel components are durable and long lasting, as shown by measurements of buildings in different climatic conditions. A design life of over 100 years is predicted for steel components contained within the building envelope. Acoustic Insulation Effective acoustic insulation of separating walls and floors between living spaces is very important to the health and wellbeing of the building occupants. For transmission of airborne sound, the acoustic performance is characterised by a sound reduction index (D nt,w in db) between rooms, based on a standard test to EN ISO 717 1, which covers a range of frequencies over 16 one third octave bands from 100 to 3150 Hz. For impact sound, which only applies to floors, the sound transmission L t,w across a floor due to a standard tapping machine should not exceed a maximum value.

6 02 Best Practice in Steel Construction - Residential Buildings Figure storey student residence constructed using a primary steel frame and light steel infill walls (Southampton, UK) Figure 2.3 Steel-framed apartment building in Evreux, France with light steel walls and floor decking and lightweight cladding Architects: Dubosc & Landowski

7 Key Design Factors 02 Table 2.1 Typical loads used in housing and residential buildings Loading Type Typical Value (kn/m 2 ) Imposed loads: Residential use 1.5 to 2.0 Corridors and communal areas 3 Commercial areas 2.5 to 4 Partitions (lightweight) 0.5 to 1.0 Self weights: Light steel walls 0.5 to 1.0 Light steel floors 0.7 Lightweight roofs 0.5 Tiled roofs 0.9 Structural steel frame 0.3 to 0.5 Composite floor slabs 2.5 to 3.5 Precast concrete slabs 2.5 to 4 For acceptable acoustic performance, the minimum airborne sound reduction is 45 db for walls and floors between separate living spaces. This performance parameter is verified by test measurements of completed buildings which also take account of local acoustic transmission through junctions, such as at floor to wall junctions. Fire Safety Fire safety in residential buildings covers a range of factors such as: effective means of escape in fire, prevention of fire spread, structural stability, and the provision of effective fire fighting measures. Requirements for structural stability and compartmentation are usually expressed as the fire resistance of the structural elements. Fire resistance is based for calibration purposes on the results of standard fire tests and is expressed in units of 30 minutes. For most housing and residential buildings, a minimum fire resistance of 30 minutes is required, increasing to 60 minutes for separating walls, dependant on national regulations. Taller buildings may require 90 minutes fire resistance primarily for reasons of structural stability and effective fire fighting. Generally, for walls and floors, the measures introduced to achieve satisfactory acoustic insulation also achieve at least 60 minutes fire resistance. Thermal Performance One of the most effective ways of reducing primary energy consumption is by improved thermal performance of the building envelope, such as by reducing thermal transmission and improving air tightness. Thermal insulation of the building envelope is characterised by its U value, which represents the heat loss through a unit area of the external elements of the façade or roof per degree temperature difference between inside and outside. A U value of 0.3 W/m 2 K is generally adopted as a maximum value for façade elements of the building envelope, and a U value of 0.2 W/m 2 K is generally adopted as the maximum for roofs (depending on the country). This can be achieved by using insulation placed externally to the light steel walls and roof so that the risk of cold bridging and condensation is minimised. One innovation is to use perforated or slotted light steel sections to reduce cold bridging effects. Most of the insulation can be placed efficiently between the light steel components and leads to a reduction in wall thickness. Loading The principal types of loading to be considered in the design of housing and residential buildings are: Self weight (including finishes). Imposed loads (including higher loads in communal areas). Wind actions. Snow loads (or roofs). Typical loads are presented in Table 2.1. Steel-framed buildings are much lighter than concrete or masonry buildings and save on foundation costs.

8 03 Best Practice in Steel Construction - Residential Buildings 03 Floor Systems This section describes the main floor systems used in housing and residential buildings. The characteristics of each floor system are described together with guidance on the important design issues. Floors may span between load bearing light steel walls, or may be supported by steel beams in a primary steel frame. There are three generic forms of floors considered in this guide: Light steel floors. Composite floor slabs. Deep composite slabs. Light steel floors are usually of C shape, although they can be of lattice form for longer spans. These lightweight flooring elements may be installed as individual components or as 2 dimensional panels (in the form of prefabricated floor cassettes). Composite slabs comprise in situ concrete placed on steel decking. Composite slabs are increasingly used in residential buildings because of their ability to provide a stiff, acoustically excellent and fire resistant construction. The steel beams are normally designed to act compositely with the slab, but in some cases, composite slabs are supported directly by light steel walls. Deep composite slabs may be of various forms using deeper deck profiles to create an overall floor depth of typically 300 mm. Beams may be integrated into the slab depth and there are no downstand beams - see Section 4. Light Steel Floor Joists Composite Floor Slabs Deep Composite Slabs Figure 3.1 Floor systems using light steel joists supported on light steel walls Fusion Building Systems

9 Floor Systems 03 Light Steel Floor Joists Figure 3.2 Light steel floor joists supported on Z sections positioned over load-bearing light steel walls Description C section joists are typically 150 to 300 mm deep and are manufactured in steel thicknesses of 1.6 to 2.4 mm using S280 to S390 galvanised steel to EN (with G275 or 40 microns total zinc coating). Lattice joists are typically 300 to 500 mm deep and permit services of up to 100 mm diameter to be passed between the bracing members. Joists are typically placed at 400 mm to 600 mm spacing to align with ceiling and floor board spans and dimensions. The floor joists are attached directly to the supporting elements or supported on Z sections that are placed over beams or walls, so that flexibility in positioning the joists is provided, as shown in Figure 3.2. When manufactured as 2 D cassettes, special attachment points are often introduced to connect the floors to the walls. Gypsum based screeds can be placed on the flooring to improve its stiffness and acoustic insulation. Steel decking can be used to replace floor boarding and achieve composite action with the joists. This form of construction is shown in Figure 3.3. For longer span areas, hot rolled or fabricated steel beams may be introduced to support the joists. These beams may be integrated in the floor depth by supporting the joists on the bottom flange of the beams, as shown in Figure 3.4. Main Design Considerations Floor joists support floor boards above and plasterboard below, which are of sufficient thickness to achieve both good acoustic insulation and fire resistance. These requirements often lead to the use of 2 or 3 layers of plasterboard in the ceiling and mineral wool or glass wool is placed between the joists. In bathrooms and kitchens, a separate servicing zone may be required below the floor, which can require use of a suspended ceiling. The light weight of these floors means that sensitivity to floor vibrations is important; the design needs to ensure that resonant effects do not occur due to walking and other normal activities. A minimum natural frequency of 8 Hz is generally adopted for lightweight floors to minimise the effect of rapid walking and other impacts on vibrations.

10 03 Best Practice in Steel Construction - Residential Buildings Figure 3.3 Lattice joists supporting gypsum screed used in long span floors Metek Building Systems Figure 3.4 Light steel floor joists supported on steel hot rolled beams Ruukki Advantages Easy to install on site. Boards attached to the joists provide acoustic insulation and fire resistance. Wide availability of different joist sizes. Floor cassettes may be manufactured and installed as larger components. Fire Resistance Fire resistance is achieved by two or three layers of fire resisting plasterboard (Type F boards to EN 520). The measures introduced for effective acoustic insulation generally achieve 60 minutes fire resistance. A fire resistance of 60 minutes is provided by 2 layers of 12 mm fire resisting plasterboard below the floor joists. Acoustic Insulation A high level of acoustic insulation is achieved using the details shown in Figure 3.5, which avoid acoustic losses at the floor-wall junctions. Various types of resilient floor covering and mineral wool placed between the joists reduce sound transmission.

11 Floor Systems 03 Flooring board Light steel separating wall and insulation Decking board Light steel separating floor Mineral wool insulation Plasterboard 300 mm min. Joints sealed with tape Figure 3.5 Acoustic build up of light steel floor and its detail at a separating wall Additional mineral wool Loads and Deflections Light steel joists support imposed loads typically up to 3 kn/m 2 for spans of 3 to 6 m (Table 3.1). Deflections should be limited to the following maximum values so that movements are not visible and to minimise perceptible floor vibrations: Span/350, or a maximum of 15 mm under self weight plus imposed load (characteristic values). Span/450 under imposed load alone. Local deflection of less than 1.5 mm under a 1 kn point load, using an effective spread of the point load onto the joists. The deflection limit of 15 mm ensures that the floor achieves the 8 Hz natural frequency limit, and leads to the maximum spans given in Table 3.1. Overall Floor Zone The overall floor zone of a light steel joisted floor, including acoustic layers and a plasterboard ceiling, is typically: 300 mm for floor spans up to 3.8 m; 400 mm for floor spans up to 4.8 m; 500 mm for floor spans up to 6 m. Floor joists Joist spacing (mm) Max. span in housing (m) Max. span in apartments (m) 150 x 1.6 C x 1.6 C x 2.0 C mm lattice joists mm lattice joists mm lattice joists with 40 mm gypsum screed Housing: Imposed loads = 1.5 kn/m 2 Self weight = 0.5 kn/m 2 Apartments: Imposed loads = 2.5 kn/m 2 Self weight = 0.7 kn/m 2 (1.7 kn/m 2 inc. gypsum screed) Table 3.1 Typical floor joist spans in housing and residential buildings

12 03 Best Practice in Steel Construction - Residential Buildings Composite Floor Slabs Figure 3.6 Typical composite slab and composite steel edge beams Kingspan Description Composite floor slabs comprise in situ concrete placed on steel decking, as illustrated in Figure 3.6. Spans of 2.5 to 4.5 m can be achieved by composite floors using steel decking of 50 to 80 mm depth with steel thicknesses of 0.8 to 1.2 mm. No temporary propping is required during construction, provided the deck depth is carefully chosen for the required span. A composite slab is typically 120 to 160 mm deep and is reinforced by mesh (such as A142 to A193, defined by the reinforcement area (in mm 2 /m)). In some cases, additional bars are placed in the rib of the decking to improve the bending resistance and fire resistance of the slab. However, 90 minutes fire resistance can be achieved by use of nominal mesh reinforcement of 0.2% of the cross sectional area of the slab. Main Design Considerations Composite slabs are relatively shallow with respect to their span (ratios of span: depth of up to 32 are possible). However, it is the span capabilities of the steel decking in unpropped construction that controls the design. For most applications, support from secondary beams or load-bearing walls is required at spans of approximately: 3 m for deck profiles of 50 mm depth; 3.6 m for deck profiles of 60 mm depth; 4.2 m for deck profiles of 80 mm depth. Design tables for composite slabs are presented in Table 3.2. Longer spans can be achieved in propped construction provided the supporting floor is capable of resisting the prop loads. Optimum design is achieved when the decking is placed continuously over one or more internal supports. Advantages Stiff, robust form of construction. Wide range of deck profiles and steel thicknesses for optimum design. No temporary propping is required for most applications. Good acoustic insulation and fire resistance. 10

13 Floor Systems 03 Fire Resistance Acoustic Insulation Load and Deflections Overall Floor Zone The effective slab depth influences the insulation that is provided in fire conditions, and so deeper slabs are required for longer fire resistance periods. The amount of reinforcement also increases with fire resistance, as its effectiveness reduces with temperature. Span and load capabilities for various slab depths and fire resistance periods for composite slabs of 120 to 150 mm depth are presented in Table 3.2. Composite floors with plasterboard ceilings can achieve excellent sound reductions of over 60 db. Load span guidance is presented in Table 3.2. In residential buildings, 80 mm deep deck profiles used in slabs of 150 mm depth can span 4.5 m without temporary propping, which is ideal for internal space planning. Deflections under imposed load are limited to span/360, but deflections of the underside of the decking after concreting can be as high as span/180. The overall floor zone of a composite floor can be as low as 250 mm allowing for acoustic layers and plasterboard ceiling, but increases due to the beam depth where steel support beams do not align with walls. In other cases where beams do not align with walls, an overall floor depth of 600 mm may be used in planning. Span Case Single span decking - no props Double span decking - no props One line of temporary props Maximum spans (m) for imposed loading Fire resistance Slab depth Reinforcement (mins) (mm) (mm 2 /m) t = 0.9 mm t = 1.2 mm 3.5 kn/m kn/m kn/m kn/m A A A A A A353* A353* A353* t = steel thickness of decking *required for crack control in propped construction A193 = 193mm 2 /m reinforcement in both directions (a) 60 mm deep decking Span Case Single span decking - no props Double span decking - no props Maximum spans (m) for imposed loading Fire resistance Slab depth Reinforcement (mins) (mm) (mm 2 /m) t = 0.9 mm t = 1.2 mm 3.5 kn/m kn/m kn/m kn/m A A A A A (b) 80 mm deep decking Table 3.2 Typical design tables for composite floors 11

14 03 Best Practice in Steel Construction - Residential Buildings Deep Composite Slabs Figure 3.7 Steel frame with deep composite floor, integrated ASB beams and infill walls Description Deep steel decking may be designed to act compositely with a concrete slab to create an overall floor depth of typically 300 mm. Spans of up to 6 m can be achieved without requiring temporary propping. The decking profile is typically 190 to 225 mm deep, depending on the product. The minimum depth of concrete over the decking is 70 to 90 mm, depending on the fire resistance requirements. The Corus Slimdek system uses either an asymmetric ASB beam or a UC/ HE section with a welded bottom plate to support SD225 deep decking, as shown in Figure 3.7. This system is widely used in the residential sector in the UK and NL, see Section 8. Edge beams may be in the form of Rectangular Hollow Sections with a welded plate for visual and detailing reasons and for their improved torsional resistance. Hoesch Additif is a deep decking system that uses bars welded transversely to the top flange of an IPE or HE section on which the top of the decking profile sits see Figure 3.8. This system is often used in car parks with spans of up to 5.5 m. The Cofradal system uses a steel tray with high density mineral wool onto which in situ concrete is placed. This floor is 200 mm deep and can span up to 6 m in residential buildings (Section 8). Main Design Considerations Deep composite slabs can span long distances and the main design consideration is the ability of the decking to support the loads during construction without requiring temporary propping. Typically unpropped spans are as follows: 225 mm deep decking 6 m span for a slab depth of 300 mm; 190 mm deep decking 5.4 m span for a slab depth of 270 mm. Additional reinforcement is required for fire resistance. Spans up to 9 m can be achieved in propped construction. For acceptable serviceability performance, span to slab depth ratios of 25 are possible with suitable reinforcement. 12

15 Floor Systems 03 Figure 3.8 Illustration of Hoesch Additif flooring system supported by steel beams Advantages Stiff, robust form of construction. Long spans (up to 6 m in unpropped construction). Good acoustic insulation and fire resistance. Shallow floor when combined with slim floor or integrated beams. Freedom in internal space planning. Fire Resistance Acoustic Insulation Overall Floor Zone For fire resistance, the following minimum requirements may be used in the scheme design of deep composite slabs with reinforcement in the deck ribs: Fire resistance (mins) Minimum slab depth over decking Minimum reinforcement per rib Minimum reinforcement in slab mm 12 mm dia. A mm 16 mm dia. A mm 20 mm dia. A252 A193 = 193 mm 2 /m reinforcement in both directions Table 3.3 Fire resistance requirements for deep composite slabs Deep composite slabs achieve excellent sound reductions of over 60 db. Special details are required at junctions between floors and walls. The overall floor zone is typically 400 to 500 mm with acoustic layers and suspended ceiling. The use of integrated or slim floor beams means that internal walls can be placed anywhere on the plan without being affected by downstand beams. 13

16 04 Best Practice in Steel Construction - Residential Buildings 04 Wall Systems This section describes the various forms of external and internal walls using light steel framing. The characteristics of each wall system are described together with guidance on the important design issues. The thermal performance of cladding systems is presented in Section 7. Walls may be designed in light steel framing as part of a load-bearing structure or as non-load-bearing elements within a primary steel frame. There are three generic forms of walls in light steel framing: Load bearing walls. Infill walls which support the façade. Separating walls and partitions. Load-bearing light steel walls may be used to support light steel floors using C section joists or floor cassettes. Alternatively, composite slabs may be supported by a perimeter C section. Load-bearing light steel walls have been used in buildings up to 8 storeys high. Infill walls are used in a primary steel or concrete structure and are designed to support the cladding and to resist wind loads. They may be prefabricated or installed as individual C sections. This same technology may be used for internal separating walls. Load-bearing Light Steel Framing External Infill Walls in Structural Frames Separating Walls and Partitions Figure 4.1 Installation of light steel infill wall in a steel frame Kingspan Architectural 14

17 Wall Systems 04 Load-bearing Light Steel Framing Figure 4.2 Platform construction of braced light steel wall in housing Fusion Building Systems Description Load-bearing walls in light steel framing use C sections of 70 to 150 mm depth and steel thicknesses of 1.6 to 2.4 mm, manufactured into two dimensional wall panels. The most common form of construction is known as platform construction in which the walls are installed using the floors as a working platform. The use of braced wall panels is shown in Figure 4.2. Forces are transferred directly through the walls and the floors are typically supported by a Z section placed over the lower wall. Wall studs (vertical C sections) are placed at 300, 400 or 600 mm spacing to align with standard plasterboard widths of 1.2 or 2.4 m. Generally, within a wall panel, the same thickness of C section is used, although multiple C sections can be detailed next to large openings or other highly loaded areas. Double layer separating walls are preferred but in some cases, single layer walls may be used, provided that services do not penetrate the wall. Load-bearing light steel walls are usually one of three generic forms: Double layer walls comprising mineral wool or glass wool insulation placed between the C sections and two plasterboard layers placed on the outer faces of the wall. Double layer walls, as above, but with rigid insulation board placed between the layers. Single layer walls using a minimum of 100 mm deep C sections with resilient bars fixed to the outer face of the C section and mineral wool between the C sections and two plasterboard layers (fastened to the resilient bars). These forms of construction are illustrated in Figure 4.3. Double layer walls are used mainly in separating walls and a total thickness of 300 mm may be used for these walls in scheme design. In other cases, single layer walls may be used and their thickness is as low as 150 mm. 15

18 04 Best Practice in Steel Construction - Residential Buildings Wall C section (Depth x width x thickness) Effective height of wall (m) Bending resistance (knm) Cross-sectional resistance Compression resistance - no buckling (kn) Buckling resistance (kn) Reduced buckling resistance allowing for eccentricity (kn) 70 x 45 x x 45 x x 45 x x 45 x Note: Reduced buckling resistances in this Table allow for the effect of the eccentricity of the axial force acting at the face of the C section. Table 4.1 Typical manufacturer s data giving compression resistance of load-bearing wall studs using C sections or 285 (a) Double layer wall - insulation between the C sections 2 x or 310 (b) Double layer wall - insulation between the layers 2 x Figure 4.3 Various forms of light steel load bearing walls (c) Single layer wall - resilient bar used to attach the plasterboard 16

19 Wall Systems 04 Main Design Considerations Load-bearing walls in light steel structures are designed for combined compression and bending due to eccentric loading transferred from the floors. For multi storey applications, 100 mm deep x 1.6 mm thick C sections are generally sufficient when placed at 300 to 600 mm spacing, but for 2 storey housing, smaller 70 x 1.2 mm C sections may be used. The compression resistance of walls using C sections is dependent on their buckling resistance, as modified for the eccentricity of axial force and for the stabilising effect of boards attached to them. For most C sections, compression resistance is governed by major axis buckling; buckling about the minor axis is restrained by mid height bracing or by attachment to facing boards. Data for the compression resistance of C section studs are presented in Table 4.1. Where vertical forces are applied at an eccentricity to the wall (e.g. for floors supported on a Z section placed over the wall panels), a reduction factor is made in Table 4.1 to account for combined bending and compression. To resist horizontal forces, walls can be braced by various methods: Integral K or W bracing using C sections acting in tension or compression. External X bracing using flat steel strips acting in tension. Diaphragm action of wall boards, such as plywood or cement particle boards. Generally, X bracing is most efficient for taller buildings and shear forces of up to 20 kn can be resisted by a 2.4 m square X braced wall panel. Various forms of cladding can be attached through external insulation used to create a warm frame, as illustrated in Section 7. Open roof structures can be manufactured using adaptations of this technology. Advantages Wall panels can be manufactured to suit any wall size and loading. Large openings can be provided for windows. Smaller wall panels (typically 2.4 m square) can be lifted manually. Large wall panels can be lifted mechanically, reducing installation time. Bracing can be installed during manufacture of the walls. Lightweight construction with no wastage of materials. Fire Resistance Acoustic Insulation The fire resistance period for load-bearing walls depends on the protection by the plasterboard. The critical temperature of load bearing wall studs may be taken as 400 C when evaluating the fire protection strategy. It is generally found that the details required for good acoustic insulation achieve at least 60 minutes fire resistance. Good airborne sound insulation of light steel walls is provided by the details shown in Figure

20 04 Best Practice in Steel Construction - Residential Buildings External Infill Walls in Structural Frames Figure 4.4 Light steel infill walls within a composite steel structure Description Non load-bearing infill walls provide support to the external envelope and are designed to resist wind forces and to support the weight of the cladding. Infill walls are one of two generic types: Individual C sections (wall studs) installed on site and placed in a bottom and top track attached to the top of the slab and the underside of the beam of slab. Prefabricated storey high wall panels that are attached externally to the structure and are connected to the columns and floors, as in Figure 4.1. An example of light steel infill walls used in a primary steel frame is shown in Figure 4.4. Infill walls can also comprise perforated or slotted C sections, as described in Section 7, which provide higher levels of thermal insulation. Some provision for relative movement between the wall and the primary structure is made at the top of the infill wall, depending on whether the structure is in steel or concrete. Brickwork is generally ground supported, or supported on stainless steel angles attached to the primary frame. Lightweight façades are usually attached to the infill wall and are supported by it. Main Design Considerations Infill walls are designed primarily for wind loading with some additional vertical load due to the self weight of the wall and its cladding. Large prefabricated panels can be designed to span horizontally between columns, as well as vertically between floors, as shown in Figure 4.4. Wind pressures are determined according to EN , depending on the building location, height and orientation. South or west facing panels at the corners of the buildings are the most critical for design. The provision for relative movement depends on the type of support, but the following minimum movements are considered reasonable for beams up to 5 m span: 10 mm for steel-framed buildings or existing concrete buildings; 20 mm for new concrete buildings. The top of the wall panel is usually restrained by a bracket attached at not more than 600 mm centres to the inside face of the panel. Each bracket is designed to resist wind suction (negative) forces and allow for relative vertical movement. 18

21 Wall Systems 04 Figure 4.5 Prefabricated wall panel with cladding and windows Ruukki Advantages Fire Resistance Acoustic Insulation Overall Wall Thickness Rapid construction system that is used with either primary steel frames or concrete frames. Lightweight construction, with minimum material use and no waste on site. Large openings can be created. Wall panels can be prefabricated or site installed. Cladding can be pre attached in prefabricated wall systems. The fire resistance of an external wall should be sufficient to prevent passage of smoke and flame from floor to floor. Normally, 30 or 60 minutes fire resistance is required, which is achieved by one or two layers of 12 mm thick fire resisting plasterboard. Special details are required at edge beams to allow for relative vertical movement. In some cases, the walls provide some fire protection to the edge beams. The acoustic insulation requirements for external walls depend mainly on the type of cladding used. Generally, an acoustic attenuation of at least 30 db is achieved by external walls with lightweight cladding. The overall thickness of external walls depends on the level of thermal insulation and type of cladding that is required. Guidance is presented in Section 7. Brickwork is usually ground supported, or supported by the structural frame. 19

22 04 Best Practice in Steel Construction - Residential Buildings Separating Walls and Partitions Figure 4.6 Plasterboard being attached to separating wall at X bracing Description Separating walls are internal walls that are required to achieve acoustic insulation between separate parts of a building or between dwellings. These walls are often also required to provide a fire compartmentation function. Separating walls can also provide a load-bearing function, as described earlier, or alternatively, are non loadbearing walls placed within a primary steel or concrete frame. Partitions are non-load-bearing walls that have no acoustic insulation or fire compartmentation function. Partitions can be removed without affecting the function of the building. Light steel C sections used in separating walls and partitions are 55 to 100 mm deep in 0.55 to 1.5 mm thick steel, depending on their height and loading. Generally, separating walls are of two forms, as illustrated in Figure 4.3: Double leaf walls with two layers of plasterboard directly fixed to the outer faces. Single leaf walls with two layers of plasterboard fixed to resilient bars that are fixed to the outer face of the C section walls. Provision for relative movement should also be made at the top of the wall in a primary steel or concrete frame. Main Design Considerations Single and double leaf walls of both types achieve these levels of acoustic performance using multiple layers of boards. Installation of a partition wall at a bracing position is shown in Figure 4.6. A double leaf wall is less sensitive to acoustic losses through service penetrations than a single leaf wall. 20

23 Wall Systems 04 Advantages Lightweight separating walls are fast to build. Excellent airborne sound reduction. All non load-bearing light steel walls are relocatable. Minimum use of materials and minimum waste on site. Self weight is less than 0.5 kn/m 2 expressed per unit floor area. Fire Resistance Acoustic Insulation Overall Wall Thickness Non load-bearing walls that meet the acoustic performance requirements also generally achieve a fire resistance of at least 60 minutes. Separating walls are designed for airborne sound reductions of typically 52 db without a low frequency correction factor C tr,, or 45 db with a low frequency correction factor. Suitable separating wall details are presented in Figure 4.3. The typical thicknesses of separating walls and particles may be taken in scheme design as: Double leaf separating wall: 300 mm. Single leaf separating walls: 200 mm. Partitions: 100 mm. 21

24 05 Best Practice in Steel Construction - Residential Buildings 05 Primary Steel Frames This section describes the various forms of structural steel components that may be used in multi storey residential buildings. The characteristics of the primary steel members are described and their combination with the floor and wall systems presented earlier. For multi-storey residential buildings requiring open plan space, a primary steel structure is the preferred option. Various steel systems are considered in this publication: Steel frame with precast concrete slabs. Composite steel frame with a composite slab. Integrated beam or slim floor construction. Inverted steel beams, such as the Slimline system. Beams in a structural steel frame are usually arranged to align with separating walls, but using integrated beams, internal walls can be located anywhere on plan and are not influenced by the depth of the downstand beams (Figure 5.1). Integrated beams may use a variety of flooring systems, including deep composite slabs, precast concrete units and light steel floor joists. Columns use HE/UC sections or Square Hollow Sections (SHS) that are usually designed to fit within the width of a separating wall. Steel Frames with Precast Concrete Slabs Composite Steel Frame with Composite Slabs Integrated Beams or Slim Floor Construction Inverted Steel Beams Figure 5.1 Steel frame using ASB sections and deep decking being installed 22

25 Primary Steel Frames 05 Steel Frames with Precast Concrete Slabs Figure 5.2 Use of precast concrete slabs placed on steel beams Description Precast concrete floor slabs are supported on the top flange of steel beams, and in some cases, may be designed to act compositely with the steel beams by use of shear connectors that are pre-welded to the top flange, as shown in Figure 5.2. In order to provide a suitable minimum bearing length, the width of the top flange should be at least 190 mm, which leads to use of deeper IPE/UB sections or HE/UC sections. Precast concrete slabs may be of two forms when used with downstand beams: Thin solid slabs ( mm thick) supporting an in-situ concrete topping and generally designed to act compositely with the steel beams. Spans are in the range of 2.5 to 4 m. Hollowcore slabs ( mm thick), which are usually designed non compositely, but can be designed compositely with a thin concrete topping. Spans are in the range of 5 to 9 m. Precast concrete slabs can also be used with integrated beams, as described later. Main Design Considerations For beams supporting precast concrete slabs, the main design consideration is that of the minimum beam width to allow for construction tolerances and, if designed compositely, for sufficient space around the shear connectors used to develop composite action. For this reason, precast slabs are generally used in long span applications using deeper or wider beams. A concrete topping (60 mm minimum) is generally required for acoustic insulation in residential buildings, and it also assists in satisfying fire resistance and robustness requirements through the mesh reinforcement in the topping. 23

26 05 Best Practice in Steel Construction - Residential Buildings Advantages Long span beams and slabs. Essentially a prefabricated construction process. Good acoustic insulation. Downstand beams can be aligned with separating walls. Fire Resistance Precast concrete slabs can achieve up to 90 minutes fire resistance without a concrete topping or up to 120 minutes with a concrete topping and with reinforcing bars embedded in the filled hollow cores. Fire protection of the steel beams can be achieved by: Board protection. Spray protection. Intumescent coatings. Acoustic Insulation Loads and Deflections Precast concrete slabs with a concrete topping or screed provide excellent airborne sound reduction. Steel beams supporting precast concrete slabs are relatively deep and can be designed for a span:depth ratio of approximately 18. Deflections will be within normal limits of span/360 under imposed loads. For scheme design, the overall floor zones given in Table 5.1 should be used for steel beams supporting precast hollowcore concrete slabs. Table 5.1 Overall floor depths for steel beams supporting hollowcore slabs Beam Span (m) Slab Span (m) Overall Floor Depth (mm)

27 Primary Steel Frames 05 Composite Steel Frame with Composite Slabs Figure 5.3 Composite steel decking and composite cellular beams Description Composite slabs are supported on the top flange of steel beams and are designed to act compositely with the beams by use of shear connectors that are generally welded through the decking as an on site process. Composite beams are widely used in all sectors of construction and are also used in residential buildings but, in this case, spans are relatively short (5 to 9 m). Composite action greatly increases the bending resistance and stiffness of the beams. Slab spans depend on the depth of the deck profile and whether the slab is propped during construction. Typically spans range from 3 m for 50 to 60 mm deep profiles to 4 to 4.5 m for 80 to 100 mm deep profiles (see Section 3). Main Design Considerations In composite construction, the main criterion is to minimise the floor depth without compromising its stiffness. For this reason, shallow HE/UC sections are often used in residential buildings to achieve spans of 5 to 9 m. Beams are contained within a suspended ceiling or are aligned with separating walls. Generally, slabs and beams are designed as unpropped in the construction condition, which means that the deck profile has to be chosen carefully to avoid use of propping see Table 3.2. Composite beams can be perforated for services, such as in cellular beams, as shown in Figure 5.3. Long span composite beams can be designed as a podium to support a light steel structure above. Advantages Stiff, relatively shallow floor. HE/UC sections can be used as beams to minimise the floor depth. Good acoustic insulation. Walls can be aligned with beams to minimise the floor depth. Long span composite beams can be designed as a podium structure over car parking. 25

28 05 Best Practice in Steel Construction - Residential Buildings B 130 A Span of slab IPE D HE 130 D (a) Plan view of floor (b) Minimum weight of profile (c) Minimum depth of profile Span of Primary Beam (B) Span of Secondary Beam (A) 6 m 8 m 10 m 12 m 5 m IPE 240 IPE 300 IPE 360 IPE m IPE 240 IPE 330 IPE 400 IPE m IPE 270 IPE 330 IPE 400 IPE m* IPE 300 IPE 360 IPE 450 IPE 550 *requires use of 80 mm deep decking and 150 mm deep slab (a) Sizes of secondary beams Span of Primary Beam (B) Span of Secondary Beam (A) 6 m 8 m 10 m 12 m 5 m IPE 270 IPE 300 IPE 330 IPE m IPE 270 IPE 300 IPE 360 IPE m IPE 300 IPE 330 IPE 400 IPE m* IPE 300 IPE 360 IPE 450 IPE 550 (b) Sizes of primary beams Table 5.2 Design tables for composite beams Fire Resistance Acoustic Insulation Load span Tables Composite slabs achieve a fire resistance of up to 120 minutes using only mesh reinforcement, provided they are designed as continuous over one or more internal spans. Additional reinforcing bars can be placed in the deck ribs in heavily loaded areas (e.g. plant rooms). Fire protection to the beams can be provided by the same measures as for precast concrete slabs. Composite slabs achieve excellent acoustic insulation provided a suitable resilient floor covering is used. The key aspect is the interface between separating walls and the steel beams in which case, the space between the deck ribs must be filled by mineral wool to avoid acoustic losses over the wall and through the slabs. Composite beams supporting composite slabs are relatively shallow and may be designed for a span:depth ratio of approximately 24. Composite beams are very stiff for control of floor vibrations. The critical design case is control of total deflections, which are limited to a maximum of span/200. The steel beam deflection is caused by the weight of wet concrete supported during construction. The load span tables in Table 5.2 may be used for composite beams with a 130 mm deep composite slab and 60 mm deep decking (except where shown by *). 26

29 Primary Steel Frames 05 Integrated Beams or Slim Floor Construction Figure 5.4 Integrated floor beams supporting precast concrete slabs Description Integrated beams (also known as slim floor beams) support a precast concrete slab or deep composite slab so that the beam and slab occupy the same depth. These sections can be of various forms: HE or UC sections with a welded bottom plate. IPE sections cut at mid height and welded to a bottom flange plate. Rolled ASB beams of asymmetric cross section. RHS with a welded bottom plate, often used for edge beams. Where integrated beams support hollowcore concrete slabs, as shown in Figure 5.4, the slabs often span a longer distance than the beams, so that the depth of the slab and beam are compatible. A concrete topping is generally used. Integrated beams are designed to achieve minimum structural depth. Main Design Considerations Integrated beams supporting hollowcore slabs are designed so that the slab spans up to 9 m and the beam spans 6 to 7.5 m. The critical design case is that of torsion acting on the beam during construction and loading due to unequal adjacent spans. Integrated beams using deep composite slabs can span up to 9 m when spaced at 6 m. Advantages Fast construction process. No limit on building height, subject to design and location of suitable bracing arrangements. Long span beams provide open plan space and freedom in internal partitioning. Integrated beams or slim floor beams minimize the floor depth. Fire Resistance For integrated beams or slim floor beams, the partial encasement of the steel section in concrete achieves up to 60 minutes fire resistance. Additional fire protection can be applied to the bottom flange by various methods, such as: Board protection, for example by plasterboard. Intumescent coatings applied on site or in the factory. Boards are most practical for columns. Intumescent coatings maintain the profile of the member and are thin (1 to 2 mm thick). These coatings can be applied off-site. 27